Field of Invention
The invention concerns the field of metallurgical heat treating. More particularly, the invention concerns processes for carburizing ferrous metal parts under a controlled atmosphere at elevated temperatures. These processes are commonly referred to as gas carburizing.
Description of Related Art
Carburization is the conventional process for the case hardening of steel. In gas carburizing, the steel is exposed to an atmosphere which contains components capable of transferring carbon to the surface of the metal from which it diffuses into the body of the part. In many carburizing processes, an important constituent of the furnace atmosphere used to carburize metal parts is the carrier gas. A carrier gas serves to provide a furnace with a positive protective atmosphere wherein an enriching gas may be dispersed.
A variety of carrier gases have been employed in carburizing as discussed in U.S. Pat. No. 4,049,472, but the most common carrier gas is the endothermic (endo) gas derived by partial combustion of natural gas in air. When using endothermic gas, it is usually necessary to add a relatively small quantity of another constituent (i.e., enriching gas), usually natural gas, to the atmosphere to raise the carbon potential of the furnace atmosphere. In most industrial processes, the endo gas is produced using an external generator. However, Barbour U.S. Pat. No. 6,159,306 discloses a device for the internal or in-situ generation of endo gas within the confines of the furnace cavity. The in-situ generator device as shown in the '306 patent is marketed by the Heavy Carbon Co. of Pittsford, Mich. under the trademark ENDOCARB™.
During operation of a furnace, using either an internal or an external generator, the carbon potential of the furnace atmosphere can become too high. When the carbon potential becomes too high it can lead to the excessive formation of carbon on the metal parts being carburized, which can cause the parts to “cement” together, and also the formation of excess carbon in the interior of the furnace (“soot”). In order to lower the carbon potential of a furnace atmosphere, it is a common practice to add air to the process chamber of the furnace. However, when the carbon potential is lowered, the time required to carburize is increased. Thus, it would be very desirable to be able to operate a furnace at a very high carbon potential and avoid the complications of excessive carbon formation or sooting.
Naito et al. U.S. Pat. No. 6,051,078 discloses a method of carburization wherein endo gas is generated inside the furnace. However, like many prior art methods, such method leads to uncontrolled sooting when the carbon potential is in excess of about 1.3%.